Transfusion of red blood cells (RBCs) has become one of the most important therapies of clinical medicine in the modern era. RBC transfusion can be life-saving in some situations, such as massive blood loss due to trauma or during surgery. The development of RBCs storage has resulted in the transfusion of more than 15 million units of RBCs every year in the United States. The FDA requires that at least 75% of the cells remain in the circulation 24h after infusion and hemolysis be less than 1% at the end of the approved storage period 1. According to this standard, most RBCs can be stored for up to 6 weeks after filtration leukoreduction in plastic bags made of polyvinyl chloride 2-4. The current 6-week RBCs storage period has worked well, with 92% of all RBC units finding a recipient before expiration 5. Though 6 week old blood meets the current FDA criteria, it is not clear whether this old blood is as safe and effective as blood stored for shorter durations. Stored blood has been shown to result in storage lesions including metabolic effects, shape change, membrane loss, rheologic changes, losses of membrane carbohydrates, oxidative injury to lipids and proteins, changes in oxygen affinity and delivery, increased adhesion of RBCs to endothelial cells, and reduced RBC lifespan as well as the secondary risks of accumulating concentrations of potassium, shedding of active proteins, lipids and microvesicles, and bacterial contaminants 6. Though in a healthy individual it is not expected (though not clear) that these changes will have a detrimental effect, it has been shown in a study of cardiac surgery patients, all of which had at least one underlying pathology, that older blood (>14 days) resulted in a higher mortality and postoperative complications than fresher blood (<14 days) 11. Despite increasing evidence in laboratory and clinical data demonstrating the deleterious effects of older blood 7-12, the conclusions drawn from these studies have been limited by design, ie, retrospective study data or have had confounding factors such as differences in transfusion volume, blood type, or patient history. Thus, we intend to use a canine model of acute blood loss to determine if there is a deleterious effect associated with transfusing old blood compared to fresh blood. We have designed a two part study. The first part of the study will develop a reproducible model of acute blood loss which is applied in the second part in a pilot, the outcome of which would be used as a guide to develop a more definitive study to determine whether there is a difference in outcome between the transfusion of old blood (5-6 weeks) vs. fresh blood (2-7 days) in a model of massive blood loss. We have completed the development of the hemorrhage model that will be used to compare the old and fresh blood. These studies will follow the completion of the comparison of old and fresh blood in a pneumonia model of critical illness.